Mis contact structure with metal oxide conductor

ABSTRACT

An electrical contact structure (an MIS contact) includes one or more conductors (M-Layer), a semiconductor (S-Layer), and an interfacial dielectric layer (I-Layer) of less than 4 nm thickness disposed between and in contact with both the M-Layer and the S-Layer. The I-Layer is an oxide of a metal or a semiconductor. The conductor of the M-Layer that is adjacent to and in direct contact with the I-Layer is a metal oxide that is electrically conductive, chemically stable and unreactive at its interface with the I-Layer at temperatures up to 450° C. The electrical contact structure has a specific contact resistivity of less than or equal to approximately 10 −5 -10 −7  Ω-cm 2  when the doping in the semiconductor adjacent the MIS contact is greater than approximately 2×10 19  cm −3  and less than approximately 10 −8  Ω-cm 2  when the doping in the semiconductor adjacent the MIS contact is greater than approximately 10 20  cm −3 .

RELATED APPLICATIONS

This is a CONTINUATION of U.S. application Ser. No. 15/451,164, filedMar. 6, 2017, which is a CONTINUATION of U.S. application Ser. No.15/186,378, filed Jun. 17, 2016, now U.S. Pat. No. 9,620,611, each ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to what are generally known in the art asmetal-insulator-semiconductor (MIS) electrical contacts, and inparticular, such contacts in which the “insulator” is an interfacialdielectric layer that is an oxide of a metal, an oxide of asemiconductor, or both, and the “metal” is a conductive metal oxide.

BACKGROUND

Metal-semiconductor contact resistivity is an important consideration inthe fabrication of field effect transistors and becomes increasinglyimportant as the dimensions of the contacts are scaled down andcurrently approach ten nanometers. MIS contacts are a relatively recenttechnological development, and may provide a contact resistivity that islower than the resistivity of the equivalent direct metal-semiconductorcontact between the same metal and semiconductor pair. As described inthe Applicant's U.S. Pat. No. 7,084,423, incorporated herein byreference, a very thin, interfacial dielectric layer between the metaland semiconductor acts to reduce the Schottky barrier at the junctionand at the same time has sufficient conductivity, despite being itself adielectric with poor bulk electronic conduction, to provide a netimprovement in the conductivity of the MIS junction.

SUMMARY OF THE INVENTION

Described herein is an electrical contact structure including aconductor; a semiconductor (e.g., a group IV semiconductor such assilicon, germanium, or an alloy mixture of silicon, germanium, carbon,or tin; a group IV compound semiconductor such as SiC; a III-V compoundsemiconductor; a II-VI compound semiconductor; a two-dimensionalsemiconductor such as graphene, phosphorene; or a transition metaldichalcogenide such as monolayer molybdenum disulfide; or carbonnanotubes); and an interfacial dielectric layer of less than 4 nmthickness disposed between and in contact with both the conductor andthe semiconductor, wherein the conductor is a conductive metal oxide,and wherein the interfacial dielectric layer is an oxide of a metal, anoxide of a semiconductor, or a mixture of both. Although the term“metal” is often used in the literature (as in“metal-insulator-semiconductor” contact), herein, we adopt the term“conductor” when referring to the conductive metal oxide in order toemphasize this nature of the contact element. In various embodiments,the electrical contact structure has a specific contact resistivity ofless than or equal to approximately 10⁻⁵-10⁻⁷ Ω-cm² when the doping inthe semiconductor adjacent the MIS contact is greater than approximately2×10¹⁹ cm⁻³ and less than approximately 10⁻⁸ Ω-cm² when the doping inthe semiconductor adjacent the MIS contact is greater than approximately10²⁰ cm⁻³. In various embodiments the interfacial dielectric layer maybe a material that is an insulator or a semiconductor in its bulk state.In some embodiments, the interfacial dielectric layer has a thickness inthe range 0.2 nm to 4 nm, and may be one of: TiO₂, SrTiO₃, MgO, Al₂O₃,HfO₂, ZrO₂, Ta₂O₅, V₂O₅, BaZrO₃, La₂O₃, Y₂O₃, HfSiO₄, ZrSiO₄, CoO, NiO,ZnO, SiO₂. The conductive metal oxide layer may, in some embodiments,have a thickness in the range 0.5 nm to 3 nm and may be one of:(Nb,Sr)TiO₃, (Ba,Sr)TiO₃, SrRuO₃, MoO₂, OsO₂, WO₂, RhO₂, RuO₂, IrO₂,ReO₃, ReO₂, LaCuO₃, Ti₂O₃, TiO, V₂O₃, VO, Fe₃O₄, ZnO, InSnO or CrO₂. Insome embodiments, the interfacial dielectric layer includes a separationlayer (e.g., a further insulating oxide layer separating the conductorand the semiconductor). Preferably, the junction between the conductivemetal oxide layer and the interfacial dielectric layer is chemicallystable up to a temperature of 400° C. and more preferably, the junctionbetween the conductive metal oxide layer and the interfacial dielectriclayer is chemically stable up to a temperature of 450° C. For example,there is preferably no chemical reaction between the conductive metaloxide layer and the interfacial dielectric layer that substantiallyconsumes the interfacial dielectric up to a temperature of 400° C. andmore preferably no such chemical reaction up to a temperature of 450° C.Also, or alternatively, the contact resistivity of the device ispreferably less than or equal to approximately 10⁻⁷ Ω-cm² after thedevice is heated to a temperature of 400° C. More preferably, thecontact resistivity of the device remains less than or equal toapproximately 10⁻⁷ Ω-cm² after the device is heated to a temperature of450° C. In some embodiments, the conductive metal oxide layer of theelectrical contact structure is contacted by a thin metal layer of adifferent metal, such as W, Ag, Al, Ta, Co, or Cr. Although examples ofmaterials that may comprise the interfacial dielectric layer arespecified herein, persons of ordinary skill in the art will appreciatethat such materials may not have the precise stoichiometry of theexamples. For instance, TiO₂ may more preferably be described asTiO_(x), with x less than or equal to 2 but greater then 1.5. Similarnon-stoichiometric variants of the metal oxides described herein,together with mixed metal oxides and mixtures of metal and silicon orgermanium oxides should be understood to be within the scope of thepresent invention. In some cases, non-stochiometric and also dopedvariants of the metal oxides exhibit semiconducting or even conductingproperties, even if their undoped stoichiometric variants are considereddielectrics.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the figures of the accompanying drawings, in which:

FIG. 1 illustrates an example of an MIS contact structure according toan embodiment of the present invention.

FIG. 2 illustrates a further example of an MIS contact structureaccording to an embodiment of the present invention.

FIGS. 3A, 3B, and 3C illustrate examples of structures created duringfabrication of an MIS contact structure according to an embodiment ofthe present invention.

DETAILED DESCRIPTION

As noted above, an MIS stack formed by a very thin, interfacialdielectric layer (termed herein an “I-Layer”) between a metal oxideconductor (or “M-Layer”) and a semiconductor (or “S-Layer”) (e.g.,silicon, germanium, silicon carbide, or germanium tin) has sufficientconductivity, despite the dielectric itself having poor conductivity inits bulk state, to provide a net improvement in the conductivity (overthat which would exist in the absence of the I-Layer) of the junction ofwhich it is a constituent. To enable a favorable result with a loweringof the contact resistivity, it is necessary for the interfacialdielectric layer, which might normally be an insulating orsemiconducting material in its bulk state, to be very thin—of the orderof one nanometer—to enable a large current density to flow, for exampleby quantum mechanical tunneling.

For the specific purpose of forming improved contacts to n-typesemiconducting materials, in which the current is carried by transportof electrons between the conductor and semiconductor, it may bepreferred to use an interfacial dielectric layer that itself presentsonly a small energy barrier to electrons. An example of such a materialis titanium dioxide, which is found to present only a very small barrierto electron flow between a metal and an n-type semiconductor, such as Sior Ge, of the order of zero to 0.15 electron volts. In general, withfavorable band alignment, the dielectric metal-oxide thickness can be asmuch as 5 nm or even 10 nm.

Whilst MIS contacts with a titanium oxide interface layer have beendemonstrated to be effective in reducing contact resistivity for a broadrange of combinations of metals and semiconductors, a problem may arisewhen MIS contacts are integrated into the industrial manufacture ofsemiconductor devices in integrated circuits (“ICs”).Metal-semiconductor contacts are processed at what is known as themiddle of line (MOL), which is the stage in integrated circuitmanufacturing that occurs after transistor fabrication (front end ofline, FEOL) and before the processing of metal interconnect layers (backend of line, BEOL). As a consequence the metal contacts are exposed tothe processing temperatures or “thermal budget” of the processes thatoccur during the BEOL, including any annealing or sintering processsteps that may be applied as part of the BEOL. In the current state ofthe art of semiconductor integrated circuit manufacturing, it is typicalfor the BEOL to involve exposure of the contacts to a temperature ofaround 400° C. over a period of approximately two hours. Such a thermalbudget may cause serious degradation of the MIS contact properties,including returning the contact resistivity to a high level moreconsistent with a direct metal-semiconductor contact. The degradation isdue most likely to a chemical reduction of the critical thin interfacelayer if it is an oxide (e.g., titanium oxide). Most metals have anaffinity for oxygen, i.e., there is a chemical driving force to form ametal oxide when the metal is in the presence of oxygen, especially atelevated temperatures. As such, most metals when placed in contact witha metal oxide such as titanium dioxide (TiO₂) and heated will removeoxygen from the titanium dioxide, rapidly reducing the TiO₂ to asub-stoichiometric titanium oxide and, upon continued heatingthereafter, to what amounts to titanium metal with a high concentrationof residual oxygen. We find experimentally, for example, that MIScontacts in which the metal is titanium, the interface layer is titaniumoxide and the semiconductor is silicon rapidly degrade totitanium-silicon contacts when heated to 400° C. for as little as 30seconds. The low Schottky barrier to n-type semiconductor and thecorresponding low contact resistivity provided by the MIS structure islost as a result of the thermal degradation of the interface layer.

We find therefore that there is a need to form an MIS contact structureof low contact resistivity that also has sufficient thermal stability tobe useful in an IC manufacturing process. The present invention involvesusing a metal oxide as the conductive metallic layer (the M-Layer) inthe MIS structure and using a different metal oxide or an oxide of asilicon or germanium as an I-Layer. The I-Layer generally comprises amaterial that would be an insulator or a semiconductor in its bulkstate, and may include a separation layer (e.g., a further insulatinglayer separating the conductor and the semiconductor).

Although most metal oxides tend to be electrically insulatingdielectrics, there are a number of metal oxides that exhibitelectrically conductive or metallic properties. Examples of electricallyconducting metal oxides (which may be suitable as conductive metallayers (M-Layers) in the present MIS structure) include but are notlimited to (Nb,Sr)TiO₃, (Ba,Sr)TiO₃, SrRuO₃, MoO₂, OsO₂, WO₂, RhO₂,RuO₂, IrO₂, ReO₃, ReO₂, LaCuO₃, Ti₂O₃, TiO, V₂O₃, VO, NbO, Fe₃O₄,conducting ZnO, InSnO and CrO₂. Examples of insulating metal oxides thatmay be used as an I-Layer in the present MIS structure include but arenot limited to TiO₂, MgO, Al₂O₃, HfO₂, ZrO₂, Ta₂O₅, V₂O₅, BaZrO₃, La₂O₃,Y₂O₃, HfSiO₄, ZrSiO₄, CoO, NiO, SrTiO₃ or (Ba,Sr)TiO₃, non-conductingZnO, MnO, silicon oxide or germanium oxide. It may be noted that somemetal oxides may exhibit a metal-insulator transition as a result of achange in temperature or pressure. For example the class of metalcompounds known as Mott-Hubbard insulators, including the transitionmetal oxides NiO, Ti₂O₃, V₂O₃, Cr₂O₃, are insulating at low temperaturesand conducting at high temperatures. Considering V₂O₃, this materialmakes an insulator-metal transition at a temperature of about 145 K atatmospheric pressure and makes an insulator-metal transition at apressure of about 7 kilobar at “room temperature”, i.e., approximately300 K. For the purposes of the present invention we are concerned withthe electrical conductivity of metal oxides in the normal range oftemperature and pressure experienced by MIS contacts during the normaloperation of semiconductor integrated circuits—temperature typically inthe range 0° C. to 125° C. and pressure close to atmospheric pressure.Non-stoichiometric variants of the metal oxides described herein,together with mixed metal oxides and mixtures of metal and silicon orgermanium oxides should be understood to be within the scope of thepresent invention. In some cases, non-stochiometric and also dopedvariants of the metal oxides exhibit semiconducting or even conductingproperties, even if their undoped stoichiometric variants are considereddielectrics.

The application of a metal oxide as a conducting or metallic layer isknown in the art of metal-insulator-metal (MIM) capacitorfabrication—for example, in the manufacture of dynamic random accessmemory (DRAM) devices—wherein a charge storage capacitor is required tohave a high specific capacitance and also to have sufficient thermalstability to survive BEOL processing. For example, U.S. Pat. No.5,619,393, incorporated herein by reference, discloses ahigh-dielectric-constant material layer is formed between a lower thinunreactive film on a lower electrode base, and an upper thin unreactivefilm with an upper electrode then formed on the upper unreactive film.In the Technical Digest of the 1994 International Electron DevicesMeeting, pp. 831-834, it is disclosed that a polycrystalline RuO₂ thinfilm is deposited by a reactive sputtering method on a TiN thin filmserving as a diffusion barrier layer and the laminate of the TiN thinfilm and the polycrystalline RuO₂ thin film is subsequently patterned todefine a composite RuO₂/TiN storage electrode before a SrTiO₃ thindielectric film is deposited on the RuO₂/TiN. The SrTiO₃ thin dielectricfilm is selected for its good chemical and thermal stability and goodinsulating properties and the purpose of the paraelectric SrTiO₃ thindielectric film is to provide the insulating dielectric layer for a DRAMcapacitor. The insulating SrTiO₃ thin dielectric film is not thin enoughto pass a current and is not in direct contact with a semiconductor.

Generally, the instant invention uses a thin, unreactive, and conductivefilm to contact the thin I-Layer material in an MIS contact structure.The thin, unreactive film may be a conductive oxide such as RuO₂ (othersinclude: (Nb,Sr)TiO₃, (Ba,Sr)TiO₃, SrRuO₃, MoO₂, OsO₂, WO₂, RhO₂, IrO₂,ReO₃, ReO₂, LaCuO₃, Ti₂O₃, TiO, V₂O₃, VO, Fe₃O₄, ZnO, InSnO or CrO₂). Inaddition to highly conductive tungsten(IV) oxide, also known as tungstendioxide, WO₂, there are other intermediate oxides of tungsten includingW₁₈O₄₉, W₂₀O₅₈, and W₂₄O₇₀ that may for example be formed by reactingtungsten metal and tungsten trioxide.

The thin, unreactive film is generally less than 10 nm thick, preferablyless than 5 nm thick, and most preferably between 0.5 nm and 3.0 nmthick. As used herein, the term “unreactive”, when used in reference toa material contacting an I-Layer material, means a material thatprovides a thermally stable conductive interface to the I-Layer materialduring and after processing (e.g., to temperatures in the range ofapproximately 350° C. or 450° C. in a forming gas or similar gas for onthe order of approximately 2 hr.). Note that when a conductive metaloxide such as RuO₂ is used for the thin, unreactive, and conductivefilm, that layer can also contain some amount of unoxidized or partiallyoxidized metal, for example Ru. For example, a thin, unreactive film ofRuO₂ which is chemically changed by becoming partially reduced duringthe deposition process of the complete metal stack and ensuing thermalcycles is still considered unreactive since it still provides a stableconductive interface to the I-Layer material. The I-Layer may be on theorder of 0.2 nm-10 nm, or more preferably 0.2 nm-5 nm, or still morepreferably, 0.2 nm-1.0 nm.

When the instant invention is applied to the fabrication of MIS contactsin integrated circuits, it is understood that several different metallayers constitute an interconnect system that is key to the operation ofthe integrated circuit. Typically, multiple highly conductive metalinterconnect layers, often as many as ten or more, are used to form thecomplex interconnections between devices and these layers are mostcommonly copper surrounded (clad) by a barrier metal such as tantalumnitride (TaN). The multiple TaN-clad copper layers are separated byinter-layer dielectric (ILD) layers, with the ILD material being areliable insulator such as silicon dioxide. The connection between thelowest copper interconnect layer and the MIS contact is typically by wayof a tungsten plug formed in a contact via. To provide good adhesion ofthe W plug to the via, the via is typically first lined with a depositedthin layer of “adhesion” and/or “barrier” metal before the W isdeposited. The adhesion or barrier metal is typically titanium nitride(TiN) or a Ti/TiN thin laminate. Referring to FIG. 1, the structure ofan MIS contact 10 according to an embodiment of the instant inventionthus includes, in sequence, a W plug 12 and a TiN barrier/adhesion layer14, an unreactive, conductive metal oxide 16 (such as RuO₂), and anI-Layer 18 (such as TiO₂, TiSi_(x)O_(y), or TiO₂/SiO₂), with the I-Layer18 being in contact with the semiconductor 20. The semiconductor 20 istypically silicon, germanium, silicon germanium alloy, germanium tinalloy, or silicon germanium tin alloy but may also be a compoundsemiconductor such as SiC, GaN, InGaN, GaAs, InAs, InGaAs alloy, GaSb orInSb, terniary or quarterniary compound semiconductors, or atwo-dimensional semiconductor such as graphene, silicene, germanene,phosphorene, monolayer molybdenum disulfide, or carbon nanotubes.

With reference to FIG. 2, the component parts of an MIS structure 22according to an embodiment of the instant invention may therefore bedescribed as follows: The M-Layer 24 is a stack of metals such asCu/TaN/W/TiN/RuO₂. The I-Layer 26 comprises one or more oxides, an oxideof titanium TiO_(x) and/or an oxide of silicon SiO_(x) (such as TiO₂ orTiSi_(x)O_(y) or TiO₂/SiO₂), that would be considered insulator(s) orsemiconductor(s) in bulk form but are conductive when made extremelythin. The S-Layer 28 is a semiconductor such as silicon or germanium orsilicon germanium alloy. An innovative aspect of the invention is thatthe metal at the bottom of the metal stack i.e., the metal that isadjacent to and in direct contact with the I-Layer is a metal oxide thatis electrically conductive, chemically stable, and unreactive at itsinterface with the I-Layer at temperatures up to 450° C. The disclosedMIS structure also has the property that the contact resistivity for thecontact structure is lower than approximately 10⁻⁵-10⁻⁷ Ω·cm², andpreferably lower than approximately 10⁻⁸ Ω·cm².

In one embodiment of the invention, illustrated in FIG. 3A, afterforming a highly n-type doped region of semiconductor 20, which may forexample be a source or drain region of a field effect transistor, theregion is covered by a deposited layer of insulating material 30, whichmay be an oxide of silicon. Subsequently, a contact window, or via, isdefined (e.g., by photolithographic or other means) and a hole is etchedthrough the insulating layer to expose a surface of the n-type dopedsemiconductor region. The exposed surface of the n-type dopedsemiconductor region is cleaned, for example by a short exposure tohydrofluoric acid diluted in water, and then a first layer of a metaloxide (I-Layer, 18) is formed on the exposed surface. Thereafter adifferent metal oxide layer 16 that is electrically conductive isdeposited over the first metal oxide layer 18. Subsequently, a layer ofan adhesion or barrier metal 14 is deposited and the contact hole isthen filled with a different bulk metal to form a metal plug 12. If anyof the processes used to form the conductive material layers is notselective, those conductive material layers will be deposited on allsurfaces including on top of the insulating material 30 as well aswithin the contact hole as illustrated in FIG. 3A. In such a case, thoseconductive material layers that are not deposited selectively within thecontact hole are subsequently removed from atop the insulating layer,for example by chemical-mechanical polishing (CMP), leaving theconductive materials substantially filling the contact hole to form anMIS contact contained within the contact hole, as shown in FIG. 3B.Subsequently, multiple layers of metal interconnect separated byinsulating material may be deposited and patterned and the wholeassembly may be annealed at temperatures in excess of 300° C., and evenat 400° C. or higher, to improve the functional properties or thereliability of the integrated circuit. Annealing may be in an atmosphereof hydrogen and nitrogen gas (so-called forming gas) or inert gas, andaccumulated annealing time may be tens of minutes and as much as twohours.

In another embodiment of the invention, illustrated in FIG. 3C, afterforming a highly n-type doped region of semiconductor 20, which may forexample be a source or drain region of a field effect transistor, theregion is covered by a deposited layer of insulating material 30, whichmay be an oxide of silicon. Subsequently, a contact window, or via, isdefined (e.g., by photolithographic or other means) and a hole or trenchis etched through the insulating layer to expose a surface of the n-typedoped semiconductor region. The exposed surface of the n-type dopedsemiconductor region is cleaned, for example by a short exposure tohydrofluoric acid diluted in water, and then a first layer of a metaloxide (I′-Layer, 18′) is formed on the exposed surface. Thereafter adifferent metal oxide layer 16′ that is electrically conductive isdeposited over the first metal oxide layer 18′. A layer of tungsten 14is deposited by metalorganic chemical vapor deposition or atomic layerdeposition and the contact hole is then filled with tungsten to form atungsten plug 12. If any of the processes used to form the conductivematerial layers is not selective, those conductive material layers willbe deposited on all surfaces including on top of the insulating material30 as well as within the contact hole as illustrated in FIG. 3A. In sucha case, those conductive material layers that are not depositedselectively within the contact hole are subsequently removed from atopthe insulating layer, for example by chemical-mechanical polishing(CMP), leaving the conductive materials substantially filling thecontact hole to form an MIS contact contained within the contact hole,as shown in FIG. 3B. Subsequently, multiple layers of metal interconnectseparated by insulating material may be deposited and patterned and thewhole assembly may be annealed at temperatures in excess of 300° C., andeven at 400° C. or higher, to improve the functional properties or thereliability of the integrated circuit. Annealing may be in an atmosphereof hydrogen and nitrogen gas (so-called forming gas) or inert gas, andaccumulated annealing time may be tens of minutes and as much as twohours.

In alternative embodiments, the process of forming the contact hole mayexpose more than one n-type doped region of semiconductor. The sameprocess may expose at least one region of p-type doped semiconductor inaddition to the at least one n-type doped region of semiconductor. Then-type doped region(s) of semiconductor may be silicon regions orsilicon-carbon alloy regions or silicon-phosphorus alloy regions orother semiconductor regions. The p-type doped region(s) of semiconductormay be silicon regions or silicon-germanium alloy regions or germaniumregions or other semiconductor regions.

In one embodiment of the invention, the n-type doped region ofsemiconductor is a silicon (Si) source or drain region of a field effecttransistor, where the silicon is very heavily doped with phosphorus (P)to a concentration in excess of 10²⁰ cm⁻³ (which may be denoted as asilicon-phosphorus alloy, Si:P) the first metal oxide layer is titaniumdioxide (TiO₂) and it is deposited by atomic layer deposition (ALD) to athickness of between 0.2 nm and 3.0 nm, and the second metal oxide layeris conductive ruthenium oxide (RuO₂) deposited by ALD to a thickness ofbetween 1.0 nm and 5.0 nm. The adhesion or barrier metal is titaniumnitride (TiN), deposited by ALD to a thickness of between 1.0 nm and 5.0nm, and the bulk metal that forms a metal plug is tungsten (W),deposited by chemical vapor deposition (CVD).

In another embodiment of the invention, the n-type doped region ofsemiconductor is a silicon (Si) source or drain region of a field effecttransistor, where the silicon is very heavily doped with phosphorus (P),for example to a concentration in excess of 10²⁰ cm⁻³ (which may bedenoted as a silicon-phosphorus alloy, Si:P) the first metal oxide layer(I-layer) is tungsten trioxide (WO₃) with a thickness of between 0.2 nmand 3.0 nm, and the second metal oxide layer (M-layer) is conductivetungsten oxide (e.g., WO₂ or a conductive tungsten oxide withstoichiometry between WO₂ and WO₃ such as W₁₈O₄₉) with a thickness ofbetween 0.5 nm and 5.0 nm. The adhesion or barrier metal is tungsten(W), deposited by metalorganic chemical vapor deposition or atomic layerdeposition to a thickness of between 0.5 nm and 5.0 nm, and the bulkmetal that forms a metal plug is also tungsten (W), deposited bychemical vapor deposition (CVD) or physical vapor deposition (PVD). Thefirst metal oxide (WO₃) is deposited by atomic layer deposition (ALD) oris formed by deposition of a thin layer of tungsten metal that issubsequently oxidized. The conductive tungsten oxide (e.g. W₁₈O₄₉ orWO₂) is deposited by ALD or CVD or is formed by reaction of tungstenmetal with the WO₃ I-layer to form a conductive tungsten oxide layer ontop of some remaining thickness of the WO₃ Mayer. The conductivetungsten oxide may have a composition intermediate between WO₂ and WO₃(including one or a mixture of WO₂, W₁₈O₄₉, W₂₀O₅₈, W₂₄O₇₀) and thecomposition may vary in this range through the depth of the conductivetungsten oxide layer. Reaction of W with WO₃ forms a sub-stoichiometrictungsten oxide layer of composition ranging between WO₂ and WO₃(including one or a mixture of WO₂, W₁₈O₄₉, W₂₀O₅₈, W₂₄O₇₀).Furthermore, one or a few atomic monolayers of silicon oxide may existat the silicon interface between the WO₃ and the silicon. The metal thatforms a metal plug may alternately be a low resistance metal, forexample copper or silver.

In another embodiment of the invention, the n-type doped region ofsemiconductor is a silicon (Si) source or drain region of a field effecttransistor, where the silicon is very heavily doped with a donor such asphosphorus (P), for example to a concentration in excess of 10²⁰ cm⁻³,the first metal oxide layer (I-layer) is V₂O₅ with a thickness ofbetween 0.2 nm and 3.0 nm, and the second metal oxide layer (M-layer) isconductive vanadium oxide (e.g., V₂O₃) with a thickness of between 0.5nm and 5.0 nm. The adhesion or barrier metal layer comprises tungsten(W) or vanadium (V) or a mixture of W and V, deposited by metalorganicchemical vapor deposition or atomic layer deposition to a thickness ofbetween 0.5 nm and 5.0 nm. The first metal oxide (V₂O₅) is deposited byatomic layer deposition (ALD) or is formed by deposition of a thin layerof vanadium metal that is subsequently oxidized. The conductive vanadiumoxide (e.g. V₂O₃) is deposited by ALD or CVD or is formed by reaction ofvanadium metal with the V₂O₅ Mayer. The conductive vanadium oxide mayhave a composition intermediate between VO and VO₂ and the compositionmay vary in this range through the depth of the conductive vanadiumoxide layer. Reaction of V from the adhesion or barrier metal layer withV₂O₅ forms a vanadium oxide layer of composition ranging between VO andVO₂ on top of some remaining thickness of the V₂O₅ Mayer. Furthermore,one or a few atomic monolayers of silicon oxide may exist at the siliconinterface between the V₂O₅ and the silicon. The metal that forms a metalplug may alternately be a low resistance metal, for example copper orsilver.

In another embodiment of the invention, the n-type doped region ofsemiconductor is a silicon (Si) source or drain region of a field effecttransistor, where the silicon is very heavily doped with a donor such asphosphorus (P), for example to a concentration in excess of 10²⁰ cm⁻³,the first metal oxide layer (I-layer) is TiO₂ with a thickness ofbetween 0.2 nm and 3.0 nm, and the second metal oxide layer (M-layer) isconductive tungsten oxide (e.g., WO₂) with a thickness of between 0.5 nmand 5.0 nm. The adhesion or barrier metal layer comprises tungsten (W),deposited by metalorganic chemical vapor deposition or atomic layerdeposition to a thickness of between 0.5 nm and 5.0 nm. The first metaloxide (TiO₂) is deposited by atomic layer deposition (ALD) or is formedby deposition of a thin layer of titanium metal that is subsequentlyoxidized. The conductive tungsten oxide (e.g. WO₂) is deposited by ALDor CVD or is formed by reaction of tungsten metal from the adhesion orbarrier metal layer with the TiO₂ Mayer. The conductive tungsten oxidemay have a composition intermediate between WO₂ and WO_(2.95) and thecomposition may vary in this range through the depth of the conductivetungsten oxide layer. Reaction of W from the adhesion or barrier metallayer with TiO₂ forms a conductive mixed oxide layer comprising amixture of tungsten and titanium oxides on top of some remainingthickness of the TiO₂ Mayer. Furthermore, one or a few atomic monolayersof silicon oxide may exist at the silicon interface between the TiO₂ andthe silicon.

In other embodiments of the invention, the first metal oxide layer(I-Layer) may comprise any one of or a combination of WO₃, TiO₂, MgO,Al₂O₃, HfO₂, ZrO₂, Ta₂O₅, V₂O₅, BaZrO₃, La₂O₃, Y₂O₃, HfSiO₄, ZrSiO₄,CoO, NiO, GaO, SrTiO₃ or (Ba,Sr)TiO₃, silicon oxide or germanium oxideor doped or non-stochiometric insulating variants thereof. Moreover thefirst metal oxide may alternatively be deposited by atomic layerdeposition (ALD), plasma-enhanced ALD, chemical vapor deposition (CVD),plasma-enhanced CVD, atomic vapor deposition, oxidation of a depositedthin film of the metal, laser ablation, physical vapor deposition, or byreaction of a metal with a thin layer of an oxide of the n-type dopedregion of semiconductor. The first metal oxide may be a mixed oxide,comprising the oxides of two different metals (such as hafnium zirconiumoxide, Hf_(x)Zr_(1-x)O₂) or comprising the oxides of a metal and asemiconductor such as silicon or germanium, such as titanium silicate,TiSi_(x)O_(y) or a mixed metal/semiconductor oxide with some amount ofnitrogen (i.e. a metal silicon oxynitride such as titanium siliconoxynitride).

In still further embodiments of the invention, the second metal oxidethat is electrically conductive (M-layer) may comprise any one of or acombination of SrRuO₃, a conductive oxide of tungsten (e.g., WO₂, W₂O₅,or a mixture of tungsten oxides with compositions ranging between WO₂and WO_(2.95) including one or a mixture of WO₂, W₁₈O₄₉, W₂₀O₅₈, W₂₄O₇₀,W₂₅O₇₃, W₄₀O₁₁₈), LaCuO₃, Fe₃O₄, LaSrCoO₃, conducting ZnO, and CrO₂, oran oxide of ruthenium (RuOx) or iridium (IrOx), a dioxide of ruthenium(RuO₂), rhodium (RhO₂), palladium, osmium (OsO₂), or iridium (IrO₂), ora conductive oxide of rhenium (ReO₂ or ReO₃), titanium (Ti₂O₃ or TiO) orvanadium (V₂O₃ or VO) or indium tin oxide (ITO) or aluminum-dopedzinc-oxide (AZO) or doped or non-stochiometric conducting variantsthereof. The second metal oxide may alternatively be deposited byreactive sputtering (of the metal in a plasma comprising oxygen), ALD,CVD, laser ablation, cyclic voltametric deposition, anodic deposition,electrophoretic deposition, oxidation of a thin film of the metal, orany of the other means discussed above.

As noted above, although examples of metal oxides that may comprise theinterface layer are specified herein, persons of ordinary skill in theart will appreciate that such metal oxides may not have the precisestoichiometry of the examples. For instance, TiO₂ may more preferably bedescribed as TiOx, with x less than or equal to 2 but greater then 1.5.Similar stoichiometries of others of the metal oxides described hereinshould be understood to be within the scope of the present invention.Furthermore persons of ordinary skill in the art will appreciate thatsuch metal oxides may include some amount of nitrogen, the amount ofnitrogen being less than the amount of oxygen.

In other embodiments the adhesion or barrier metal may be tantalumnitride or ruthenium oxide or tungsten or CoWP. The adhesion or barriermetal may be deposited by atomic layer deposition or chemical vapordeposition (CVD) or metal organic chemical vapor deposition (MOCVD) orphysical vapor deposition (PVD). The bulk metal that forms a metal plugmay be cobalt or copper or aluminum or silver or a plurality of carbonnanotubes. The metal that forms a metal plug may alternatively bedeposited by CVD, PVD, sputtering or electrochemical deposition.

A similar process may be applied to form MIS contacts to p-type dopedregions of semiconductor material.

In various embodiments, the layers described herein may be deposited ina single process chamber (in sequential processing steps), in differentchambers of a multi-chamber processing tool, or in separate processingtools.

Thus, described herein is an electrical contact structure including aconductor; a semiconductor (e.g., a III-V compound semiconductors or asemiconductor comprising one or several of the group IV elementssilicon, germanium, carbon, or tin); and an interfacial dielectric layerof less than 4 nm thickness disposed between and in contact with boththe conductor and the semiconductor, wherein the conductor is aconductive metal oxide, and wherein the interfacial dielectric layer isan oxide of a metal or an oxide of a semiconductor or an oxide ofmultiple different metals or metal(s) and semiconductor(s). In variousembodiments, the electrical contact structure has a specific contactresistivity of less than or equal to approximately 10⁻⁵-10⁻⁷ Ω-cm² whenthe doping in the semiconductor adjacent the MIS contact is greater thanapproximately 2×10¹⁹ cm⁻³ and less than approximately 1×10⁻⁸ Ω-cm² whenthe doping in the semiconductor adjacent the MIS contact is greater thanapproximately 10²⁰ cm⁻³. Alternatively, or in addition, the interfacelayer is a material that would be an insulator or a wide band gapsemiconductor in its bulk state. In some embodiments, the interfacialdielectric layer has a thickness in the range 0.2 nm to 10 nm,preferably 0.2 nm to 4 nm, and may be one of: WO₃, TiO₂, MgO, Al₂O₃,HfO₂, ZrO₂, Ta₂O₅, V₂O₅, BaZrO₃, La₂O₃, Y₂O₃, HfSiO₄, ZrSiO₄, CoO, NiO,GaO, SrTiO₃ or (Ba,Sr)TiO₃, silicon oxide or germanium oxide. Theconductive metal oxide layer may have a thickness in the range 0.5 nm to3 nm (although in other embodiments different thicknesses may be used)and may be one of: WO₂ (or a mixture of tungsten oxides withcompositions ranging between WO₂ and WO_(2.95) including one or amixture of WO₂, W₁₈O₄₉, W₂₀O₅₈, W₂₄O₇₀, W₂₅O₇₃, W₄₀O₁₁₈), (Nb,Sr)TiO₃,(Ba,Sr)TiO₃, SrRuO₃, MoO₂, OsO₂, WO₂, RhO₂, RuO₂, IrO₂, ReO₃, ReO₂,LaCuO₃, Ti₂O₃, TiO, V₂O₃, VO, Fe₃O₄, zinc oxide (ZnO), indium tin oxide(ITO), aluminum-doped zinc-oxide (AZO), InSnO or CrO₂. In someembodiments, the interfacial dielectric layer includes a separationlayer (e.g., a further insulating oxide layer separating the conductorand the semiconductor). Preferably, the junction between the conductivemetal oxide layer and the interfacial dielectric layer is chemicallystable up to a temperature of 400° C. and more preferably chemicallystable up to a temperature of 450° C. For example, there is preferablyno chemical reaction between the conductive metal oxide layer and theinterfacial dielectric layer that substantially consumes the interfacialdielectric up to a temperature of 450° C. Also, or alternatively, thecontact resistivity of the device is preferably less than or equal toapproximately 10⁻⁷ Ω-cm² after the device is heated to a temperature of400° C. More preferably, the contact resistivity of the device remainsless than or equal to approximately 10⁻⁷ Ω-cm² after the device isheated to a temperature of 450° C. In some embodiments, the conductivemetal oxide layer of the electrical contact structure is contacted by athin metal layer of a different metal.

What is claimed is:
 1. An electrical contact structure including aconductor; a semiconductor and an interfacial dielectric layer disposedbetween and in contact with both the conductor and the semiconductor,wherein the conductor is a conductive metal oxide, and the interfacialdielectric layer has a thickness in the range 0.2 nm to 4 nm andcomprises one of: WO3, TiO2, MgO, Al2O3, HfO2, ZrO2, Ta2O5, V2O5,BaZrO3, La2O3, Y2O3, HfSiO4, ZrSiO4, CoO, NiO, GaO, SrTiO3, or(Ba,Sr)TiO3.
 2. The electrical contact structure of claim 1, wherein theconductive metal oxide has a thickness in the range 0.5 nm to 3 nm andcomprises one of: WO2, (Nb,Sr)TiO3, (Ba,Sr)TiO3, SrRuO3, MoO2, OsO2,RhO2, RuO2, IrO2, ReO3, ReO2, LaCuO3, Ti2O3, TiO, V2O3, VO, Fe3O4, ZnO,indium tin oxide (ITO), aluminum-doped zinc-oxide (AZO), InSnO, or CrO2.3. The electrical contact structure of claim 1, wherein a junctionbetween the conductive metal oxide and the interfacial dielectric layeris chemically stable up to a temperature of 450° C.
 4. The electricalcontact structure of claim 1, wherein the semiconductor is a III-Vcompound semiconductor.
 5. The electrical contact structure of claim 1,wherein the semiconductor is a semiconductor comprising one or severalof the group IV elements silicon, germanium, carbon, or tin.
 6. Theelectrical contact structure of claim 1, wherein the semiconductor is asilicon source or drain region of a field effect transistor, saidsilicon is doped n-type to a concentration in excess of 10²⁰ cm−3, theinterfacial dielectric layer is V₂O₅ with a thickness of between 0.2 nmand 3.0 nm, and the conductive metal oxide is conductive vanadium oxidewith a composition intermediate between VO and VO2 and a thickness ofbetween 0.5 nm and 5.0 nm.
 7. The electrical contact structure of claim1, wherein the semiconductor is a silicon source or drain region of afield effect transistor, said silicon is doped n-type to a concentrationin excess of 10²⁰ cm⁻³, the interfacial dielectric layer is TiO2 with athickness of between 0.2 nm and 3.0 nm, and the conductive metal oxideis conductive tungsten oxide (e.g., WO₂) with a thickness of between 0.5nm and 5.0 nm.
 8. A method of forming an electrical contact structure,comprising depositing by one of atomic layer deposition, chemical vapordeposition, or physical vapor deposition, to a thickness in the range0.2 nm to 4 nm, an interfacial dielectric layer on a semiconductor andthereafter forming a conductor on the interfacial dielectric layer suchthat the interfacial dielectric layer is thereby disposed between and incontact with both the conductor and the semiconductor, wherein theconductor is a conductive metal oxide and the interfacial dielectriclayer comprises a metal oxide.
 9. A method of forming an electricalcontact structure, comprising depositing and subsequently oxidizing ametal layer on a semiconductor, and subsequently forming a conductivemetal oxide conductor over the oxidized metal layer such that theoxidized metal layer forms an interfacial dielectric layer disposedbetween and in contact with both the conductor and the semiconductor andhas a thickness in the range 0.2 nm to 4 nm.
 10. The method of eitherclaim 8 or claim 9, wherein the interfacial dielectric layer comprisesTiO2.
 11. The method of either claim 8 or claim 9, wherein theinterfacial dielectric layer comprises V2O5.
 12. The method of claim 10,wherein the conductive metal oxide comprises conductive tungsten oxideand the conductive tungsten oxide is formed by reaction of a tungstenmetal layer with the TiO2 interfacial dielectric layer and has acomposition intermediate between WO2 and WO2.95.
 13. The method of claim11, wherein the conductive metal oxide comprises conductive vanadiumoxide and the conductive vanadium oxide is formed by reaction ofvanadium metal with the V2O5 interfacial dielectric layer and has acomposition intermediate between VO and VO2.
 14. The method of claim 10,wherein the conductive metal oxide is a conductive mixed oxide layercomprising a mixture of tungsten and titanium oxides formed by thereaction of tungsten metal layer with the TiO2 interfacial dielectriclayer on top of some remaining thickness of the TiO2 interfacialdielectric layer.